Method of and apparatus for the control of electrical currents



J. M. MILLER 1,787,732

METHOD OF AND APPARATUS FOR THE CONTROL OF ELECTRICAL CURRENTS Jan. 6, 1931.

Filed July 24, 1925 Patented Jan. 6, 1931 PATENT OFFICE UNITED {STATES JOHN M. MILLER, OF WASHINGTON, DISTRICT OF COLULIBIA, ASSIGNOR TO ATWATER -KENT MANUFACTURING COMPANY, OF PHILADELPHIA, PENNSYLVANIA, A COB- I'OR ATION OF PENNSYLVANIA METHOD OF AND APPARATUS FOR THE CONTROL OF ELECTRICAL CURRENTS Application filed July 24, 1923. Serial No. 658,490.

While my invention relates broadly to the control of electrical currents, it has as a particular object control of high frequency electrical currents such as those encountered in radio signaling and wired radio.

A particular object is the control or elimination of reaction of output circuits on input circuits of relay and amplifying devices, such as the controlling of the reactions of the plate circuit on the grid circuit of 3-electrode vacuum tubes.

A further object is the stabilization of amplifying devices and circuits to prevent self-oscillation, or to control the extent of self-oscillation or regeneration.

A still further object is an improvement in the selectivity of amplifying circuits.

My invention will be best understood by reference to the figures of the accompanying drawing which illustrate and explain it with particular reference to radio amplifier circuits used with 3-electrode vacuum tube amplifying devices.

Figurespl, 2 and 4' are explanatory illustrations, and Figures 3, 5, 6, 7 and 8 are illustrative of several ways in which my invention may be used.

Figure 1 shows the usual circuit connections of the first vacuum tube of a radio frequency amplifier. The circuits S, comrising inductance L and variable condenser may be the secondary tunable circuit of a radio receiver which is customarily con- Gr and filament F of- Batteries B and B nected between the grid the vacuum tube VT.

are the usual" filament and plate batteries encountered in such devices. The impedance Z in the plate circuit of the tube represents a coil or transformer which serves as of theamplifier. operated over a the coupling umt with the succeeding tube The amplifier is normally band of frequencies about that frequency for which the impedance Z, including such capacities as are contributed bythe leads, tubes and so forth, is resonant. At resonance, the impedance Z behaves as a ure resistance for alternating currents, for requencies lower than the resonant fre: quency the impedance Z has an inductive component and for frequencies higher than tions are also given'in frequency 1 reaction of input circuit the resonant frequency it has a capacitive component. By reason of the capacity between the plate P and grid G of the vacuum tube, there is a reaction between the plate circuit of the tube and the input circuit S and the nature of this reaction depends upon the character of the load impedance Z. This effect has been treated mathematically and experimentally by me in a paper entitled Dependence of the inputimpedance of a three-electrode vacuum tube upon the load in the plate circuit, published as Scientific 'laper No. 351 of the Bureau of Standards,

and also treated mathematically by H. W. Nichols in the Physical Review, vol. 13, p. L05, 1919. The results of these investigathe book by H. J Van Der Bijl entitled The thermionic vacuum tube on page 205 and following. These show that when the load impedance Z is a pure resistance, or has a capacitive component, the the plate'circuit upon the'input circuit S is such as to take power from the thus increasing its effective resistance and also to increase the apparent capacity in the input circuit. When the load impedance Z has an lnductive component the reaction is such as to tend to feed power from the plate circuit of the tube into the input circuit, thus reducing the apparent resistance of is still the same tendency to increase the apparent capacity in said input circuit. Either of these effects'can be detrimental to the operation of the amplifier. An increase in the apparent resistance of the circuit S will lead to broadness of tuning and loss of signal strength. On the other hand, if the power fed back from the plate circuit to the circuit S exceeds a certain amount, the tube will self generate oscillatlons and the usefulness the input circuit while there nated. Figure 2 is an explanatory diagrammatic sketch corresponding to Figure 1, the dots marked F, G and P representing the vacuum tube elements. The circuit S and impedance Z correspond to Figure 1. The capacity C drawn in dotted lines from P to G represents the capacity between the grid and plate electrodes of the tube. It has been shown that the reaction in the plate cir-' cuit of a vacuum tube can be simulated by the asumption of a resistance R and alternating elec-tromotive force in series inside the tube between the plate and filament as shown by R and E in dotted lines in Figure 2. It is evident that the alternating potential E will produce an alternating potential difference between plate P and filament F, the phase and amplitude of this potential depending upon impedance Z. As a result of the capacity between plate P and grid G there will be an alternating electromotive force applied to the circuit S. It is this electromotive force which represents the reaction of the plate circuit on the input circuit. Asstated before when the load Z is capacitive or resistive, the reaction is such as to increase the effective resistance of the circuit S, when this load is inductive, the reaction can be such as to decrease the efi'ective resistance and can lead to theself generation of oscillations, circuit S usually being of low resistance and therefore readily oscillatory.

Figure 3 shows diagrammatically one form of my invention which permits theeliminatron or control of these reactions. In this diagram the circuit of Figure 1 is changed by the addition of the capacities C and 0. connected inseries from the plate P to the filament F, the circuit S being connected from the grid G to a branch point'whi'ch is the common junction of these two capacities. Since this new mode of connection would insulate the grid for continuous currents, the leak resistance R is connected from grid G to filament F. Normally R will be a high resistance. Figure 4 is an explanatory representation of the circuit of Figure 3, drawn in a manner similar to explanatory Figure 2. The capacity C between the id electrode and filament electrode, including that of leads, terminals, and other parts,is shown in dotted l1nes. The circuit of Figure 4 is similar to an alternating current Wheatstone bridge for comparing the capacity of condensers. Neglecting the resistance R the bridge-can be balanced by giving capacities C and C the pro er values. In such a case there would be noe ectromotive force impressed upon the c1rcu1t S due to the reactive electromotive force E. The balance of the bridge being independent of the frequency, this would hold for all frequencies. Thus the circuit S would not be affected by the load in the plate circuit of the tube ;its resistance would neither be increased when the load is resistive or' capacitive nor decreased when the load is incillating it is, of course, not necessary to obtain a perfect balance but merely to reduce the reaction from the plate circuit by the requisite amount. necessary to take into account the resistance R or dielectric losses in balancing the bridge, though by putting the proper resistance in parallel or in series with the condensers C and C 3. perfect balance could be obtained It is normally not.

for precision work. The above assumes that R is of the order of a megohm as usually used with receiving tubes; in the case of power tubes,'however, R mi ght be of the order of several thousand ohms and then it would usually be. desirable to obtain a more perfect balance as by shunting G with the proper re sistance.

The received si nal which is impressed upon the circuit S must impress a voltage between the grid and filament of the tube. It, in Figure 4, the capacity C is equal to C only one half of the voltage across the tuning condenser of the circuit S will be impressed upon the tube input, therefore it is desirable to make C, greater than C The same principle can beapplied to control or eliminate the reaction between succeeding tubes in an amplifier as has been described for the first tu e. In general the most difliculty with the self-generation of oscillations is met with in the first tube, but reactions between succeeding tubes can lead to reduced amplification or it may be desired to introduce reactions which lead to an enhanced amplification. In Figure 5 is shown one, circuit in which the invention is applied between the stages of'a multistage amplifier which utilizes reactance or resistance coils as coupling units. The impedance Z represents the coupling unit between vacuum tubes VT and VT; The impedance Z .is a choke coil which has a natural frequency below the frequency range of the amplifier so that it behaves v as a capacity reactance throughout that range. By the natural frequency of choke coil Z' is meant the fre-' ous figures. C,; is the customary gri coupling condenser and R, the usual grid leak resistance. To control regeneration C and C, can be made variable. The terminal of C, which is connected to the positive plate ing potential, and specifically, filament or cathode ter bus.

he circuit in the case of a transformer type amplifier is somewhat simpler. and is shown in Figure 6. Here T and T represent coupling transformers and the capacities C and C, and resistance R, correspond with the previous figures, as well as do other'reference letters. 1

The sameprinciple is applicable to the case where a vacuum tube is employed as a coupling between the primary and secondary circuits of a receiver. Stability of such a circuit can readily be obtained and reaction of the secondary upon nated orcontrolled as desired. 1

It is apparent that partial or complete balance of the bridge of Figure 4 can also be obtained by other means which however seem less desirable. In place ofthe capacities C and 0;, either resistances or inductances can beemployed, a by-pass condenser being em ployed to prevent a short circuit of the plate battery.

In Figure 7 there is shown a' circuit employing resistances. The represent the terminals to which the input circuit is connected, while the pointsC and D are the terminals to which apparatus in the output circuit may be connected. The resistances R and It, replace the capacities C and C of the previous figures, while the capacity C, serves as a by-pass for the oscillations but prevents a short circuit of the plate battery B Similarly, Figure 8 shows a circuit when inductances are employed, L and L being substituted in place of C and C, respectively. In these lastltwo cases the resistance R, will, in general, be unnecessary since the grid will usually be connected to the filament through the circuit connected to points A and B.

It is characteristic of one of the aspects of my invention as hereinbefore described that by means of the condensers, as C C resistances R R or inductances L L connected in series with each other between the cathode or filament F and a point at which exists a varying potential, in phase with the disturbbetween the F and the anode P or other suitable point in the anode circuit, there is defined a point, as O,'at which the potential is different from that "of cathode F and varies in phase with the disturbing potential; between the point 0 and the grid G there is zero difference of potential as regards reacs tion produced by the disturbing potential; and between the point 0 and the grid is im-' by or through the input element. The input element, as circuit S or equivalent, has its one terminal connected to the point whereby in efiect the reactions produced by the primary elimipoints A and B term input element or any equivalent means, per se,.commonly.

O and its other to the rid,

the disturbingpotential produce substantially zero difference of potential between the terminals of the input element.

In accordance with another aspect of my invention, I utilize a true Wheatstone bridge in no arm of which is included an part of the input element, as of the circuit or equivalent, and in whose conjugate conductor M is connected the input element, as the circuit S or equivalent, which, and the entire conjugate conductor with reference to the cathode F and whose potential caused by the disturbing potential rises and falls throughout in phase with the disturbing potential.

It is further characteristic of my invention that the capacity imparted by connections, leads, etc., is in one arm of the bridge; and more particularly, there may be included in the same arm with the capacity) a resistance, as the grid leak resistance and similarly, there may be utilized in either or both of the arms including the capacities C and C, series or parallel resistances.

Commonly in the prior art the input terminals to a vacuum tube have been the grid and filament terminals thereof, whereas in accordance with my invention the input terminals are those of the grid and the aforesaid point, such as O, and with respect to these terminals in accordance with my invention, the amplifying apparatus operates uni-laterally, in the sense that reaction is reduced or entirely prevented as between the source of disturbing potential and the input element connected to these input terminals.

For brevity in the appended claims, the includes a circuit, as S,

heretofore used in the art for transferring to or impressing upon the grid or input circuitthe energy or electro-motive-force to be amplified or, in general, to affect the impedance of the anode circuit.

Having described my invention I claim:

1. Radio receiving apparatus comprising cascaded thermionic tubes, impedance in the anode circuit of one of said tubes. a connection from intermediate point of said impendance to the anode circuit of the following tube, an impedance in said connection, and connections from the input electrodes of said second tube to said first impedance on opposite sides of said intermediate point.

M, are at thesame potential 2. Radio receiving apparatus comprising cascaded thermionic tubes, impedence in the anode circuit of one of said tubes, :1. connection from an intermediate point of said impedance to the anode circuitv of the following tube, capacity in shunt to the portion of sald impedance common to the anode circuits of said tubes, and impedance in said connection, and connections from the input eleccuit of the following tube and a point between said impedances, and connections from trodes of said second tube to said first impedance on opposite sides of said intermediate point.

3. Radio receiving apparatus comprising cascaded thermionic tubes, impedance in the anode circuit of one of said tubes, a connection from an intermediatepoint of said impedance to the anode circuit of the following tube, a condenser in said connection, and connections from the input electrodes of said second tube to said impedance on opposite sides of said point, said condenser and capacities between said input electrodes and between the grid and anode of said second tube forming three capacitative arms of a Wheatstone bridge, including the input and output circuits of said second tube in conjugate conductors thereof.

4. Radio receiving apparatus comprising.

cascaded thermionic tubes, a coupling impedance and a capacitative lmpedance in series in the anode circuit of one of said tubes, a

said tube and between the grid and plate thereof comprising the four capacitative arms ofa Wheatstone bridge whose conjugate conductors include said input element and the output circuit of said tube.

8. An amplifier utilizing a thermionic tube, and a plurality of capacitative impedances forming an alternating current VVheatstone bridge whose conjugate conductors comprise respectively the input and output circuits of said amplifier, characterized by the fact that at least one of said impedances is an inductance having a natural period below the frequency range of said amplifier to afford a direct current path between the cathode and another electrode of said tube.

JOHN M. MILLER.

condenser connected between the'anode cirthe input electrodes of said tube to said coupling impedance, said condenser, said capacitative impedance, and capacities between said input electrodes and between the grid and anode of said second. tube forming the four capacitative arms of a Wheatstone bridge whose conjugate conductors include said coupling impedance and the output circuit of said second tube.

5. In an amplifier, the combination with a thermionic tube having an input circuit,

an inductance having a natural frequency lower than the frequency range of said amplifier connected between a point in said input circuit and the cathode of said tube, a condenser connectedbetween said point andthe anode circuit of said tube, and an' input element connected between said point and the grid of said tube.

6. An amplifier, comprisin cascaded thermionic tubes, a coupling in uctance and an inductance having a natural frequency lower than the frequency range of 'said amplifier traversed in succession by anode current of one of said tubes, a condenser connected between the anode circuit of the following tube and a point between said inductances, and connections from the input electrodes of said second tube to said coupling inductance.

' 7. In an amplifier, the combination with a i thermionic tube having an input circuit, an

inductance having a natural frequency lower than the frequency range of said amplifier connected betweena point in said, input circuit and the cathode of said tube, a condenser connected between said point and the anode circuit of said tube, and an input element connected between said point and the grid of said tube, said inductance, said condenser, and capacities between the input electrodes of 

